(a) Field of the Invention
The present invention relates to a semiconductor light emitting device. More particularly, it relates to a terbium borate-based yellow phosphor, a preparation method thereof, and a semiconductor light emitting device incorporating the yellow phosphor which absorbs a portion of light emitted by a light emitting diode, and emits light of wavelength different from that of the absorbed light, thereby implementing such white light as purely white light and bluish white light by incorporating the yellow phosphor.
(b) Description of the Related Art
A semiconductor light emitting diode (LED) is a PN-junctioned compound semiconductor. It is a kind of optoelectronic device that emits light energy corresponding to the band gap of a semiconductor generated by a combination of an electron and a hole when a voltage is applied.
As full colorization of LED was realized with the development of high luminance blue LED using a GaN-based nitride semiconductor fluorescent material, application of LEDs are expanding from display devices to illumination devices. LEDs offer about 10 to 15% less power consumption compared with conventional illumination devices such as fluorescence bulbs and incandescent bulbs, semi-permanent life of over 100,000 hours, and environmental friendliness, when used for illumination devices, so that they can significantly improve energy efficiency. For a semiconductor light emitting diode to be used for illumination purpose, white light should be obtainable using LEDs. Largely, three methods of fabricating white semiconductor light emitting devices have been used. One of them is to obtain white light by combining three LEDs of red, green and blue colors. In this method, an InGaN or AlInGaP phosphor is used as a fluorescent material. According to this method, it is difficult to constructing three RGB LEDs on a single chip and it is difficult to control a current strength because each LED is made from different material and by different method, and driving voltage of each LED is different. In another method, a UV LED is used as a light source to excite a three-color (RGB) phosphor to obtain white light. It uses an InGaN/R,G,B phosphor as a fluorescent material. This method is applicable under a high current and improves color sensation. However, the above two methods have the following problems: a satisfactory material to obtain green light has not been developed as yet; and light emitted from the blue LED may be absorbed by the red LED to lower the overall light emitting efficiency. As an alternative method, a blue LED is used as a light source to excite a yellow phosphor to obtain white light. In general, an InGaN/YAG:Ce phosphor is used as a fluorescent material in this method.
When a phosphor is used, it's emitting efficiency increases as a difference in wavelengths of an exciting radiation and an emitted radiation gets small. Thus, the light emitting characteristic of a phosphor plays a very important role in determining the color and luminance of a semiconductor light emitting device incorporating thereof. Generally, a phosphor includes a matrix made of a crystalline inorganic compound and an activator that converts the matrix into an effective fluorescent material. It emits light mainly in the visible wavelength region when an electron excited by absorbing a variety form of energies returns to its ground state. The color of emitted light can be adjusted by controlling the combination of the matrix and activator.
Examples of white semiconductor light emitting devices are disclosed in many documents.
U.S. Pat. Nos. 5,998,925 and 6,069,440 (Nichia Kagaku Kogyo Kabushiki Kaisha) disclose a white semiconductor light emitting device using a nitride semiconductor, which comprises a blue light emitting diode containing the nitride semiconductor represented by the formula: IniGajAlkN (0≦i, 0≦j, 0≦k, i+j+k=1) and a yellow phosphor containing a YAG (yttrium, aluminum, garnet)-based garnet fluorescent material that absorbs a portion of light emitted from the blue light emitting diode and emits light of wavelength different from that of the absorbed light. For the YAG-based phosphor, a mixture of a first phosphor, Y3(Al1-SGaS)5O12:Ce, and a second phosphor, RE3Al5O12:Ce, (0≦s≦1; RE is at least one of Y, Ga and La) are used.
U.S. Pat. No. 6,504,179 (Osram Optosemiconductors GmbH) discloses a white-emitting illuminating unit using a BYG approach (combination of blue, yellow and green) instead of the conventional RGB approach (combination of red, green and blue) or BY approach (combination of blue and yellow). This white-emitting illumination unit has an LED emitting a first light in the range of 300 nm to 470 nm as a light source, and the first light is converted into light of longer wavelength by the phosphor exposed to the first light. To aid the conversion, a Eu-activated calcium magnesium chlorosilicate-based green phosphor and a Ce-activated rare earth garnet-based yellow phosphor is used. For the Ce-activated rare earth garnet-based yellow phosphor, a phosphor represented by the formula RE3(Al, Ga)5O12:Ce (RE is Y and/or Tb), at least 20% of the total emission of which lies in the visible region of over 620 nm, is used.
U.S. Pat. No. 6,596,195 of General Electric discloses a phosphor which is excitable between the near UV and blue wavelength region (ranging from about 315 nm to about 480 nm) and has an emission peak between the green to yellow wavelength region (ranging from about 490 nm to about 770 nm), and a white light source incorporating the same. This phosphor has a garnet structure and is represented by the formula: (Tb1-x-yAxREy)3DzO12 (A is selected from the group consisting of Y, La, Gd and Sm; RE is selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu; D is selected from the group consisting of Al, Ga and In; A is selected such that A is different from RE; x is in the range from 0 to 0.5; y is in the range from 0.0005 to 0.2; and z is in the range from 4 to 5).
As described above, conventional white semiconductor light emitting devices excite YAG-based yellow phosphors to emit light mainly using UV to blue LEDs and obtain white light from combination thereof. However, the YAG-based yellow phosphor emits yellowish green light, and if other materials are added in place of yttrium and aluminum to cause a change in emitted light toward a longer wavelength, the emitting luminance is reduced.